Band-pass selector system



Jan. 2, 11940. H. A. WHEELER ABl-xND-PSS SELECTOR SYSTEM Filed July 29,1957 www INVENTOR HAROLD A* WHEELER ATTORNEY Patented Jan. 2, 1940UNITED STATES PATENT GFFICE BAND-PASS SELECTOR SYSTEM poration ofDelaware Application July 29, 1937, Serial No. 156,295

19 Claims.

This invention relates to high-frequency bandpass selector systems and,more particularly, to such systems in which the width of the frequencyband transmitted by the system is adjustable. While band-pass selectorsystems constructed in accordance with the present invention are ofgeneral utility, they are particularly suitable for use 1n theintermediate-frequency channel of superheterodyne receivers forcontrolling theselectivity and delity of response of the receiver.

Band-pass selector systems of conventional design usually comprise apair of suitably coupled resonant circuits tuned to the same or slightlydiiferent frequencies. The two circuits are commonly referred to asinput and output, or primary and secondary, circuits depending uponwhich circuit is connected to the source of current of the highfrequencies comprising the band to be transmitted. The responsiveness ofthis type of selector system is usually substantially constant over aband of frequencies in the vicinity of resonance of the individualcircuits, while currents of all other frequencies are sharply disy,criminated against and are attenuated to a substantial degree. Ingeneral, the width of the frequency band passed by such system may bevaried either by changing the coupling between the two circuits or byadjusting the resonant frequencies of the two circuits relative to eachother. The coupling referred to is nondirective in nature; that is,either circuit may be made the input circuit and the other the outputcircuit, without substantially affecting the characteristics of thesystem. Such system is to be distinguished from a system including Vavacuum-tube coupling between the input and output circuits, in which thecoupling is mainly unidirective.

' Band-pass selector systems wherein only the nondirective form ofcoupling is employed are,

in general, open to the criticism that only mehanical or relativelycomplicated nonmechanical expediente are known for adjusting the widthof the frequency band to be transmitted. Furtherincre, this type ofcoupling is inherently incapable pf producing amplification of thetransmitted frequencies in the coupling path between the input andoutput circuits of the system. Band-pass selector systems of the priorart, wherein a vacw uum tube or a undirective coupling is used in both.the forward and backward directions between the input and outputcircuits, are open to the criticism that a device having a directivetransconductance is required for each of the directive coupling paths.

It is an object of the present invention to provide an improvedadjustable band-pass selector system of simple arrangement, in which thecoupling between the input and output circuits of the system is obtainedby a single device having directive transconductance.

It is another object of the invention to provide a band-pass selectorsystem of the type described, in which a single vacuum tube controls theamplication between the terminal circuits and, in addition, controls theresonant frequencies of the terminal circuits to adjust the width of theband of frequencies passed by the system.

It is a further object of the invention to provide an improved band-passselector system of the type described, which is adjustable, by means ofthe conventional automatic amplification control of the receiver inwhich the system is embodied, to procure ideal band-pass characteristicsfor a wide range of received signal amplitude.

Briefly stated, the above objects are obtained in accordance with thepresent invention by providing a band-pass selector system comprisingresonant input and output circuits coupled by directive coupling meansin the forward direction and having means for introducing a reactancecomponent in each of the terminal circuits which varies with thetransconductance of the coupling tube effectively to alter the resonantfrequencies of the terminal circuits in accordance with thetransconductance of the coupling tube. With this form of couplingbetween the terminal circuits, the dependence of both the amplificationbetween the terminal circuits and the width of the band of frequenciespassed by the system on the transconductance of the coupling tube makesit possible simultaneously to adjust the width of the frequency bandtransmitted and the gain of the system. The band-pass characteristic issymmetrically varied with respect to the mean frequency of the system byadjustment of the transconductance of the coupling tube. The form ofcharacteristic obtained is that resulting from symmetrical detuning ofthe input and output tuned circuits as described in the paper entitledHigh fldelity receivers with expanding selectors, by Wheeler andJohnson, published in the I. R. E. Proceedings, June, 1935, at pages594-609, with the added desirable feature that the gain of the circuitfurther decreases with decreasing selectivity.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, willbest be understood by reference to the following description, taken inconnection with the accompanying drawing, and its scope will be pointedout in the appended claims. In the drawing, Fig. 1 is a simplifiedcircuit diagram of a band-pass selector system embodying the invention;Fig. 2 is a circuit diagram, partly in block form, of a completesuperheterodyne receiver embodying a band-pass selector constructed inaccordance with the invention; Fig. 3 illustrates certain operatingcharacteristics of the band-pass selector system shown in Fig. 2; andFigs. 4, 5, 6, and 7 illustrate diierent arrangements for neutralizingthe inherent capacitance of the vacuum tube of Figs. 1 and 2.

Referring to Fig. 1 of the drawing, there is illustrated one form of theimproved band-pass selector system, comprising a tuned input circuit L1,C1 and a tuned output circuit L2, C2 coupled by a vacuum tube l0 in theforward direction. For the purpose of effectively coupling a reactanceinto circuit L2, C2 which varies directly in accordance with thetransconductance of the tube I0, there is provided a tuned circuit La,C3 Which is included in the input circuit of vacuum tube I0, in serieswith the tuned circuit L1, C1 and moderately inductively coupled to thecircuit L2, C2, as indicated by the dotted line M23. Since the purposeof circuit L3, C3 is not to obtain selectivity, but rather to obtain acurrent having a phase displacement of one quadrant with respect to thecurrent of circuit La, C2, the tuning of this circuit to the meanresonant frequency of the selector system is broadened by a shuntresistor R3 of relatively small resistance. Similarly, for the purposeof effectively coupling a reactance of opposite type into the primarycircuit L1, C1 which varies directly in accordance with thetransconductance of the vacuum tube I0, there is provided a tunedcircuit L4, C4 which is included in the output circuit of the vacuumtube IIJ in series with the tuned circuit Lz, C2 and moderatelyinductively coupled to the circuit L1, C1, as indicated by the dottedline M14. Circuit L4, C4 is also very broadly tuned to the mean resonantfrequency of the selector system, the tuning being broadened by a shuntresistor R4 of relatively small resistance. The input or primary circuitL1, C1 is sharply tuned to a frequency slightly above or below the meanresonant frequency of the selector system. The output or secondarycircuit L2, Cz is sharply tuned to a frequency differing equally andoppositely from circuit L1, C1 with relation to the mean resonantfrequency to be passed by the selector system. In order to coupleopposite reactance 1 components into circuits L1, C1 and La, C2, the

mutual inductances M14 and M23 are of opposite polarity. The simplestarrangement is one which is symmetrical with respect to input and outputcircuits, that is, L1, C1, La, C3, and R3 are equal, respectively, toL2, Cz, L4, C4, and R4. In this case, M14 and M23 are equal andopposite. Operating potentials for the vacuum tube I0 may be supplied byany suitable source, such as the batteries II, I2, and I3, while thegrid bias may be adjusted by means of a voltage divider II'.

'Ihe adjustment of feed-back circuits Lc, Ca and L4, C4 is made beforethe terminal circuits are adjusted. If one of the feed-back circuits hasa phase displacement more or less than one quadrant with respect to theterminal circuit to which it is inductively coupled, the feedbackeffects not only the tuning but also the damping of the terminalcircuit. The phase of the feedback circuits Ls, Ca and L4, C4 may beadjusted by slight variation of C3 and C4, respectively. The phase ofeach is adjusted so as to have no eiect on the damping of the sharplytuned circuit with which it is inductively coupled. The

' only effect of circuit La, Ca is then an effect on the resonantfrequency of circuit Lz, C2; likewise for L4, C4 and L1, C1. Theadjustment of circuits L1, C1 and LzfCz can then easily be effected bytuning these circuits to the mean resonant frequency of the selectorsystem with tube I0 adjusted for maximum transconductance. Reducing thetransconductance of the tube III has the effect of letting circuits L1,C1 and Lz, Cz fall out of tune by the proper amount.

In considering the operation of the system described, it will be assumedthat, initially, the transconductance of the tube I0 is a minimum, thecircuit L1, C1 is resonant at a frequency slightly above, and circuitL2, C2 is resonant at a frequency slightly below, the mean resonantfrequency of the band to be passed. Under these conditions the gain andselectivity of the system are minimum, providing the maximum fidelity ofreproduction. If the bias on tube I0 is now changed by means of anadjustment of potentiometer II' to increase the transconductance of thetube, the couplings between circuits L1, C1 and L4, C4 andA betweencircuits L2, C2 and La, C: are correspondingly increased and increasingreactance components of opposite kinds are effectively coupled into theterminal circuits L1, C1 and Lz, C2, tending to bring these terminalcircuits into tune with each other and with the mean resonant frequencyof the selector system, in this manner narrowing the band of frequencieswhich is transmitted by the system.

In Fig. 3 there is illustrated a family of characteristic curves whichare representative of the selector system of Fig. 1 when thetransconductance of the tube is varied over a range of values. It isapparent that the increase in amplification, due to the resonant gain ofthe terminal circuits and the gain of tube I 0, is cumulative as thetransconductance is increased by an adjustment of the grid bias of thetube I0. 'I'he characteristic curves are symmetrical and control of theselectivity is accomplished without complicated mechanical parts andwithout any additional tubes.

In Fig. 2 there is shown, partly in block form, a completesuperheterodyne receiver of a conventional design embodying the presentinvention in a preferred form. In general, the receiver includes atunable radio-frequency amplier I4 of one or more stages having itsinput circuit connected to an antenna I5 and ground I6 and its outputcircuit connected to a frequency changer I'I. Connected in cascade withthe frequency changer I1, in the order named, are a selector system I8,a detector and automatic ampliflcation control rectifier I9, anaudio-frequency amplifier 20 of one or more stages, and a soundreproducer 2|. Automatic amplification control is applied in aconventional manner from rectifier I9 to one or more stages ofradio-frequency amplifier I4, frequency changer I'I, and selectorcircuit I8.

It will be understood that the various system components just described,except the selector I8, may be of conventional construction andoperation, the details of which are Well known in the art, renderingfurther description thereof u unnecessary. Considering briey theoperation of the receiver as a whole, a desired received signal isselected and amplifiedy by radio-frequency amplier I4, converted to amodulated intermediate-frequency carrier in frequency changer lll,amplied and selected by band-pass selector il, and rectified by thedetector I8, thereby deriving the audio frequencies of modulation, thesefrequencies, in turn, being amplified in the audio-frequency amplifier20 and reproduced by sound reproducer 2l. 'I'he automatic amplificationcontrol bias, supplied to one or more stages of radio-frequency amplierI4, frequency changer Ill, and the band-pass selectoi system i8, servesto maintain the signal output within a relatively narrow range for awide range of received signal intensities.

The band-pass selector system I8 of Fig. 2 is similar in many respectsto the band-pass selector system illustrated in Fig. 1 and correspondingparts are identified by the same reference characters. Selector systemI8 comprises an input circuit L1, C1, C"1 inductively coupled to winding22 in the output circuit of frequency changer il and an output circuitL2, C2 inductively coupled to winding 23 in the input circuit of theunit I9 including the detector and automatic amplification controlrectifier. Tuned circuits le, Ca, C3 and L4, C4 differ from those shownin Fig. 1 only in that they are included in the cathode sides of theirrespective circuits rather than in the grid and plate sides as in Fig.1, and that condenser C: of Fig. 1 is divided into two portions Ca andCa for ease of adjustment in aligning the input and output circuits andadjusting the neutralization, as described hereinafter.

lt is believed that the operation of band-pass selector i8 will beapparent from the explanation given with respect to the selector ofFig. 1. Variation of the trans-conductance of tube 4I effected by theautomatic amplification control causes the selectivity of the circuit tovary directly with the gain and results in a family of characteristiccurves for selector i3, as shown in Fig. 3.

The inherent capacitance across the input leads and terminals of vacuumtube Ill was neglected in Fig. 1, but in Fig. 2 is represented bycondenser 2t shown in dotted lines, while the inherent capacitanceacross the output leads and terminals of the tube is represented bycondenser 25, also shown in dotted lines. While there is no appreciableundesirable capacitive coupling between terminal circuits L1, C'1, C1and L2, C2 due to the grid-anode capacitance of tube lli, because of theinterposed screen grid, the inherent capacitances represented bycondensers 24 and 25 have an undesirable coupling effect between certainof the circuits, such coupling effect between the terminal circuitsbeing most detrimental since these circuits are sharply tuned. There aretwo paths by which the terminal circuits are coupled outside of the tubel0. In the first path, terminal circuit L1, C1, C1 is coupled to thetuned circuit La, C's, C3 by the inherent input capacitance of tube lll,represented by condenser 24, while the tuned circuit comprisinginductance L3, Cs, is coupled to the tuned circuit La, C2 by the mutualinductance M23 In the other coupling path, the terminal circuit L1, C'1,C1 is coupled to the tuned circuit L4, C4 by the mutual inductance M14,while the latter tuned circuit is coupled to the terminal circuit L2, C2by the output capacitance of tube il), represented by condenser 2t. Eachof these paths involves one capacitive coupling and one inductivecoupling. If the stage is symmetrical, the coupling eiects of the twopaths are equal except that the mutual inductances are of oppositepolarity. That is, the undesirable coupling effects of the two externalpaths neutralize each other. If the system is not precisely symmetrical,or the inherent capacitances 24 and 25 are not equal, neutralization ofthe coupling effect can be effected by adjustment of either or both ofthe auxiliary condensers 26 and 21 connected, respectively, acrosscapacitances 24 and 25.

A neutralizing arrangement of three condensers is shown in the inputcircuit of tube I0, comprising capacitance C1, capacitance C3, and con--denser 26 supplementing inherent capacitance 24'.

A similar arrangement may alternatively be used in the output circuit oftube I0 or in both the input and the output circuits. In order to avoiddetuning the circuits L1, C1, C"1 and La, Cz, C"a by adjustment ofcondenser 26, the adjustable condensers C"1 and C"a may be adjustedoppositely to condenser 26 by means of a unicontrol mechanism U tomaintain constant the total capacitance effective across L1 and La.

Fig. 4 shows an alternative arrangement by which the effect ofcapacitance 24 can be neutralized entirely in the input circuit. Thearrangement is equally applicable to the output circuit of the tube forneutralizing the inherent capacitance 25. In Fig. 4, a voltage from aneutralizing coil 28, closely inductively coupled to coil L1 is coupledto the circuit La, Ca through a neutralizing .condenser 29, as shown,the values of elements 28 and 29 being such that the coupling providedthereby between circuits L1, C1 and La, Cs is equal and opposite to thatprovided by capacitance 24. The inductance of 28 may be less than thatof L1 and the capacitance of 29 greater than the inherent capacitance24'. Fig. differs from Fig. 4 only in the inversion with respect to theterminals of the input circuit, Fig. 4 being applicable to Fig. 1 andFig. 5 to Fig. 2.

Figs. 6 and 7 illustrate two other arrangements for neutralizing theincidental capacitance of the input circuit of vacuum tube I0 and dieronly in the manner of connecting the two tuned circuits in the inputcircuit of tube I0, Fig. 6 being applicable to Fig. l and Fig. 7 to Fig.2. In these modifications the circuit L1, C1 is effectively connected inseries with one-half of circuit La, C3, by means of a connection betweeninductance L1 and the mid-tap of inductance L3. The values of La, C3, R3in this case will differ from those of Figs. 1 and 2 because of thechange in the method of their connection into the input circuit of tubeI0. A condenser 3l), having a capacitance equal to that of condenser 24,couples the circuits L1, C1 and L3, C3 in opposite phase toI condenser24, as shown, and effects neutralization since the coupling eiects ofcondensers 24 and 30 between circuits L1, C1 and La, C3 are equal andopposite. As in the case of Figs. 4 and 5, neutralizing arrangementssimilar to Figs. 6 and 7 may be employed also to neutralize theincidental capacitance in the output circuit of the tube In analternative form of the invention, the sharply resonant circuits L1, C1and La, C2 are tuned to the mean frequency of the band with minimuminstead of maximum transconductance of tube l0. The resonance curves ofFig. 3 then have the same shape, but the gain at the mean frequencytends to be approximately uniform, being maximum for an intermediatevalue of transconductance. This results from the fact that the couplingtube I0 tends to increase the gain of the system with an increase initstransconductance, while the resonant gain or the system decreaseswith the symmetrical detuning which occurs with increasingtransconductance. 'I'he effect of symmetrical detuning of coupled tunedcircuits alone is shown in Fig. 3b at page 603 of the above-mentionedWheeler and Johnson paper. A selector system having such nearly uniformgain can be utilized where it is desired to change the band width of thesystem with little or no change in amplification in the controlled stageof the system. In this case, the bias on tube I 0 from the rectifier ofunit I9 has to be rearranged to provide a less negative value forgreatersignal strength in order to expand the pass band of selector i8.l

In another alternative form, the inductive couplings M14 and M23 havelike polarity so that the resonant frequencies of the sharply resonantcircuits L1,C1 and L2,C2 are shifted in the same sense by varying thetransconductance of the tube. The neutralizing arrangements of Fig. 2

then become inapplicable, but those of Figs. 4 to 7 are stillapplicable.

While there have been described what are at present considered thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modications may be madetherein without departing from this invention', and, therefore, it isaimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of this invention.

What is claimed is:

1. A band-pass selector system for passing a Vband of frequencies,comprising tuned terminal circuits, repeater means having a variabletransconductance coupling said circuits in one direction, and means foreffectively coupling into said terminal circuits reactances variable inaccordance with said transconductance, thereby to vary the selectivityof said system.

2. A band-pass selector system comprising a repeater having a variabletransconductance, input and output tuned circuits connected with saidrepeater, and two feed-back means exclusive of said tuned circuits butcoupled respectively therewith and both including said repeater, wherebysaid feed-back means couple effectively into said tuned circuitssubstantial impedance varying in accordance with the transconductance ofsaid repeater.

3. A band-pass selector system comprising a repeater having a variabletransconductance, a tuned circuit and an auxiliary circuit connectedbetween the input terminals of said repeater, a tuned circuit and anauxiliary circuit connected between the output terminals of saidrepeater, feed-back means including said repeater anda coupling betweensaid output auxiliary circuit and said input tuned circuit, and separatefeed-back means including said repeater and a coupling between saidinput auxiliary circuit and said output tuned circuit, said feed-backmeans being proportioned to couple effectively into the respective tunedcircuits substantially pure reactance variable in accordance with thetransconductance of said repeater.

4. A band-pass selector system for passing a band of frequencies,comprising tuned terminal circuits normally tuned, respectively,slightly above and below the mean resonant frequency of the band to bepassed by said system, repeater means having a variable transconductancecoupling said circuits in one directiomaand means for eectively couplinginto said terminal circuits reactances of opposite kinds varyingdirectly in accordance with variations of said transconductance, saidreactances being effective to tune said terminal circuits toward saidmean resonant frequency.

5. A band-pass selector system comprising. a repeater having a variabletransconductance, two tuned circuits equally sharply selective anddetunedequally on opposite sides of the mean frequency to be passed bysaid system, an auxiliary circuit and one of said tuned circuitsconnected between the input terminals of said repeater, an auxiliarycircuit and the other of said tuned circuits connected between theoutput terminals of said repeater, feed-backmeans including saidrepeater and a coupling between said output auxiliary circuit and saidinput tuned circuit, and separate feed-back means including saidrepeater and a coupling between said input auxiliary circuit and saidoutput tuned circuit, said feed-back means being proportioned to coupleeilectively into the respective tuned circuits substantially purereactances of opposite kinds variable in accordance with thetransconductance of said repeater and being proportioned to bring saidtuned circuits into resonance with each other at said mean frequencywhen the transconductance of said repeater ls a maximum.

6. A band-pass selector sys'tem for passing a band of frequencies,comprising terminal circuits normally tuned to the mean resonantfrequency of the band vto be passed by said system, repeater meanshaving a variable transconductance coupling said circuits in onedirection, and means for eiectively coupling into said terminal circuitsreactances oi opposite kinds variable in accordance with saidtransconductance, thereby to vary the selectivity of said system.

7. A band-pass selector comprising a repeater having avariabletransconductance, two tuned circuits equally sharply selectiveand tuned to the mean frequency'to be passed by said system, anauxiliary circuit and one of said tuned circuits connected between theinput terminals of said repeater, an auxiliary circuit and the other ofsaid tuned circuits 'connected' betweenthe output terminals of saidrepeater, feed-back means including said repeater and a. couplingbetween said output auxiliary circuit and said input tuned circuit, andseparate feed-back means including said repeater and a coupling betweensaid input auxiliary circuit and said output circuit, said feed-backmeans being proportioned to couple eiectively into the respective tunedcircuits substantially pure reactances of opposite kinds variable inaccordance with the transconductance of said repeater and beingproportioned to detune said tuned circuits equally on opposite sides of'said mean frequency.

8. A band-pass selector system for passing a band of frequencies,comprising tuned terminal circuitsa first auxiliary circuit coupled withone of said terminal circuits, a second auxiliary circuit coupled withthe other of said terminal circuits, repeater means coupling saidterminal circuits in theforward direction, a iirst feed-back pathincluding said repeater means coupling said one of said terminalcircuits to said first circuit and a second feed-back path includingsaid repeater means coupling said second circuit to the other of saidterminal circuits, and means for varying said repeater means, thereby tovary the selectivity of said system.

9. A band-pass selector system for passing a band o'i frequenciescomprising a pair of tuned terminal circuits, a first auxiliary circuitin series with one of said terminal circuits and .coupled with the otherof said terminal circuits, a second auxiliary circuit in series withsaid other of said terminalcircuits .and coupled with said one of saidterminal circuits, repeater means coupling said terminal circuits in theforward direction, a first w feed-back path including said repeatermeans coupling said one of said terminal circuits to said secondcircuit, a second feed-back path including said repeater means couplingsaid first circuit to the other of said terminal circuits, and means forvarying said repeater means, thereby to vary the selectivity of saidsystem.

I10. A band-pass selector system for passing a band of frequenciescomprising a pair of sharply tuned terminal circuits, a first broadlytuned .w circuit in series with one of said sharply tuned circuits andcoupled with the other of said sharply tuned circuits, a second broadlytuned circuit in series with said other sharply tuned circuit andcoupled with said one of said sharply tuned cir- .2u cuits, repeatermeans coupling said sharply tuned circuits in the forward direction, afeed-back path including said repeater coupling said one of said sharplytuned circuits to said second circuit, a second feed-back path includingsaid repeater am coupling said rst circuit to the other of said sharplytuned circuits, and means for varying said repeater means, .thereby tovary the selectivity of said system.

1i. A band-pass selector system for passing a band of frequenciescomprising a pair of tuned terminal circuits, a rst auxiliary circuit inseries with one of said terminal circuits and coupled with the other ofsaid terminal circuits, a second auxiliary circuit in series with theother of said terminal circuits and coupled with said one of saidterminal circuits, a vacuum-tube coupling means coupling said terminalcircuits, coupling said one of said terminal circuits to said secondcircuit, and coupling said first circuit to the other of said terminalcircuits, and means for varying the transconductance of said vacuumtube, thereby to vary the selectivity of said system.

12. A band-pass selector system for passing a band of frequenciescomprising tuned terminal circuits, a first auxiliary circuit coupledwith one of said terminal circuits. a second auxiliary circuit coupledwith the other of said terminal circuits, repeater means coupling saidterminal circuits in the forward direction, a feed-back path includingsaid repeater coupling said second circuit to the other of said terminalcircuits. said feed-back coupling path introducing incidental capacitivecoupling between said second auxiliary circuit and said one of saidterminal circuits, means for neutralizing the effect of said incidentalcoupling,.and means for varying the transconductance of said repeatermeans, thereby to vary the selectivity of said svstem.

13. A band-pass selector system comprising a to the` other of saidterminal clrcuits saidcoupling means introducing incidental capacitivecoupling between one-of said terminal circuits and its `series-connectedauxiliarycircuit, means for varying the transconductance of saidrepeater,

thereby `to vary the selectivity of said system, and

means for neutralizing the effect of said incidental capacitivecoupling.

14. A band-pass selector system comprising a pair of tuned terminalcircuits, a rst auxiliary circuit in series with one of said terminalcircuits and coupled to the other of said terminal circuits, a secondauxiliary circuit in series with the other of said terminal circuits andcoupled to said' one of said terminal circuits, repeater means having avariable transconductance coupling said terminal circuits in the forwarddirection, a feed-back circuitA including said repeater coupling saidone of said terminal circuits to said second circuit, a feed-back pathincluding said repeater coupling said' first circuit to the other ofsaid terminal circuits, said repeater means introducing incidentalcapacitive coupling between the series-connected circuits of each pair,said first and second circuits being coupled to said terminal circuitswith opposite polarity, whereby said vincidental couplings, at least inpart, neutralize each other, and means for varying saidtransconductance; thereby to vary the selectivity of said system.

l5. A band-pass selector system comprising a pair of tuned terminalcircuits, a. rst auxiliary circuit in series with one' of said terminalcircuits and coupled to the otherof said terminal circuits, a secondauxiliary circuit in series with the other of said terminal circuits andcoupled to said one of vsaid terminal circuits, said couplings to saidterminal circuits being opposite polarity, repeater means having avariable transconductance and input and output terminals coupling saidterminal circuits in the forward direction, a feedback circuit includingsaid repeater coupling said one of said terminal circuits to said secondcircuit, a feed-back circuit including said repeater coupling said firstcircuit to the other of said terminal circuits, said repeaterintroducing incidental capacitance across said input and outputterminals, means for varying said transconductance, thereby to vary theselectivity of said system,

and means comprising supplemental capacitance in parallel with thelesser of said incidental capacitances and said couplings between saidfirst and second circuits and said terminal circuits to neutralize theincidental capacitive coupling ofl said system.

16. A band-pass selector system comprising tuned terminal circuits, afirst auxiliary circuit coupled to one of said terminal circuits, asecond auxiliary circuit coupled to the other of said terminal circuits,a vacuum-tube repeater having an incidental capacitance across its inputelectrodes for coupling said terminal circuits in the forward direction,a feed-back circuit including said repeater coupling said one of saidterminal circuits to said first circuit, a feed-back circuit includingsaid repeater coupling said second circuit to the other of said terminalcircuits, means to vary the coupling effect of said coupling means,thereby to vary the selectivity of said system, and an auxiliarycoupling between said one of` said terminal circuits and said secondcircuit offjo'pposite phase to the coupling effect of said incidentalcapacitance for neutralization thereof.

17. A band-pass selector system comprising tuned terminal circuits, arst auxiliary circuit coupled to one of said terminal circuits, a secondl auxiliary circuit ,coupled to the other of .said terminal circuits, avacuum-tube repeater having incidental capacitance across its outputelectrodes for coupling said terminal circuits in the forward direction,a feed-back circuit including said repeater coupling said oneof saidterminal circuits to said rst circuit, a feed-back `circuit includingsaid repeater for coupling said second circuit to the other of saidterminal circuits, means to vary the coupling eiect of said repeater,thereby to vary the selectivity oi said system, and an auxiliarycoupling between said other of said terminal circuits and said firstcircuit oi' opposite phase .to the coupling eiect of said incidentalcapacitance for neutralization thereof. I

18. A band-pass selector system comprising a pair of tuned terminalcircuits. a iirst auxiliary circuit having a-portion in series with saidone of saidterminal circuits and coupled to the other of said terminalcircuits. a second auxiliary circuit connected in series with said otherof said terminal circuits and coupled to said one of said terminalcircuits, repeater means having a variable transconductance couplingsaid terminal circuits in the forward direction| a feed-y back circuitincluding said repeater coupling said one of said terminal circuits tosaid second circuit, a feed-back circuit including said repeatercoupling' said rst circuit to the other of said terminal circuits, meansfor varying said trans- 2,1s5,ass

conductance, thereby to vary the selectivity oiy said system, incidentalcapacitive coupling between said one oi said terminal circuits and Isaidseries-connected portion of said first circuit, and an auxiliarycoupling of opposite phase between the remaining portion of said iirstcircuit and 'said one of said terminal circuits to neutralize saidincidental capacitance coupling.

19. A band-pass selector system comprising a pair oi' tuned terminalcircuits. a iirst auxiliary circuit in series with one of said terminalcircuits and coupled to the other of said terminal circuits to saidsecond circuit. a feed-back circuit including saidrepeater coupling saidiirst circuit tothe other of said terminal circuits, means i'or varyingsaid transconductance, thereby to vary the selectivity of said system,inci-l dental capacitive coupling between the other of said terminalcircuits and said series-connected portion of said second circuit, andan auxiliary coupling of opposite phase between the remaining portion ofsaid second circuit and said other of said terminal circuits toneutralize said incidental capacitive coupling. v

HAROLD A. WHEELER.

